The machine may be, for example, a co-ordinate measuring machine (CMM), machine tool or robot etc, and the probe is a measuring probe with a workpiece-contacting stylus. One type of machine has measuring devices for measuring the movement of the machine parts in three nominally orthogonal directions (referred to as X,Y and Z axes), and one type of probe includes measuring transducers for producing outputs indicative of the displacement of the tip of the stylus relative to the probe in three nominally orthogonal directions (referred to as the a,b, and c axes). Although the term ‘analogue probe’ is used, the outputs for the a,b and c axes may be either analogue or digital.
In known systems, measurement errors are caused by unwanted deflections of the probe, machine structure and workpiece. Errors due to bending of the probe stylus are the same throughout the machine volume and may be compensated for by probe calibration. Errors due to deflections in the machine structure may be caused, for example, by the machine quill bending and the machine bridge twisting and vary throughout the machine volume. These errors increase, for example, with increasing cantilevers. Errors in the object to be measured may be caused by object deflection during measurement as a result of force by the probe.
Parts of the working volume of the machine may be calibrated for measurement errors by using a calibration artefact such as a calibration sphere. However usually the calibration artefact cannot be located at the same position in the machine's working volume as the part to be measured and is instead located to one side. The measurement errors determined at the calibration artefact will thus be different to those on the part.
Methods of correcting machines for acceleration induced errors are known. One example of such a method is described in European Patent No. 318557. In this method a first article from a batch of nominally identical articles is measured at a relatively slow speed, noting the measurements of the positions of a number of datum points on the article. The measurement operation is repeated at a relatively fast speed noting the measurements of the positions of the same datum points. Any difference in the measurements are noted as errors in a correction table.
Thereafter all of the articles are measured at the relatively fast speed taking measurements of the positions of corresponding points on each article, and these measurements are corrected for machine accelerations using the previously noted errors.
Another method of correcting errors is disclosed in WO00/62015. In this method a stylus of a probe mounted on a coordinate measuring machine is driven into contact with a surface of an object in a direction normal to its surface until a predetermined stylus deflection has been reached. The machine is then reversed whilst simultaneously recording the outputs of the machine measuring devices and measuring transducers of the probe. This process is repeated for a selection of datum points around the surface of the object. The measurements for each datum points are extrapolated to determine the measurement which would have been taken when the probe deflection is zero. This extrapolated value relates to when the probe is just in contact with the surface.
The object is then scanned at a slow speed and predetermined stylus deflection. The difference at the datum points between the initial measurements and the scan is recorded.
The scans are repeated at the same stylus deflection at greater speeds until the variation in the recorded differences between the fast scan and the initial measurements and the slow scan and the initial measurements exceeds a defined tolerance. The last speed which falls within this tolerance is the maximum scanning speed. A map of the positional errors at the datum points is stored along with the data relating to the scanning speed, particular artefact, particular CMM and probe and stylus configuration etc. It is possible to interpolate from this map to obtain radial errors at angles in between the radial directions at which actual data was obtained (i.e. the datum points).
This method has the disadvantage that the step of collecting the datum points is time consuming.